1. Field of the Invention
The present invention relates generally to semiconductor wafer testing equipment and methods, and, in particular, to an apparatus and method for retaining a semiconductor wafer on a chuck surface.
2. Description of the Related Art
The manufacturing process of semiconductor wafers generally includes the so called "front end" stage which includes the actual processing of the wafer, and the so called "back end" stage which includes the testing of the completed wafer. During the back end testing operations the wafer is normally secured to the chuck of an automated testing station. A probe card which is attached to a probe card holder is positioned above the wafer and is lowered so that its pins can stick into the wafer to make electrical contact with the pads of the particular chip(s) being tested. It is important that the wafer be tightly held in place so that as the probe card moves, its pins connect to the proper pads of the wafer. If the wafer is permitted to move, even slightly, the probe card's pins may not make contact with the proper pads, thus causing the testing operations to fail and possibly even cause damage to the wafer.
Vacuum chucks have typically been used to hold wafers tightly in place during testing operations. Specifically, a vacuum chuck includes a series of apertures in the surface of the chuck to which a vacuum source is connected. The suction created between the surface of the chuck and the bottom surface of the wafer securely holds the wafer in place.
Vacuum chucks perform very well for solid wafers through which air cannot pass. However, a vacuum chuck will not work well for a wafer having one or more "through holes". This is because a suction force between the surface of the chuck and the bottom surface of the wafer cannot be achieved since the air will continue to pass through the through holes in the wafer.
For example, semiconductor microphones and pressure sensors often include open areas which extend completely through the wafer, as well as very thin and sensitive areas, such as diaphragms. Because the open areas go all the way through the wafer, there is no way to pull a complete vacuum. Furthermore, other problems may arise in trying to use a vacuum chuck with these structures. Specifically, the vacuum may create too much pressure (atmospheric pressure) on the diaphragm and cause it to distend or even break. If the diaphragm distends without breaking it will be stretched to its limits and be unable to vibrate. A diaphragm that cannot vibrate freely in response to pressure waves cannot be properly tested.
Thus, there is a need for an apparatus and method for tightly holding micromachined wafers having through holes and/or sensitive areas in place during back end testing procedures. Furthermore, it would be desirable if such apparatus and method would be capable of being installed on a conventional vacuum chuck.